1. Field of the invention
This invention relates to an acceleration sensor and, more particularly, to a sensor for magnetically detecting an acceleration using a permanent magnet supported by a beam or deformable member which is deformed in accordance with the acceleration exerted thereon.
2. Description of the Related Art
An acceleration sensor having a beam which is bendable or deformable according to an acceleration exerted thereon is known. One of such known sensors comprises a cantilever beam having one end fixed to a base, a weight rigidly secured to the other end of the beam, an electrode attached to the base at the same level as the weight, and another electrode fixed on the base. The weight is moved and the beam is bent in accordance with an acceleration exerted on the weight. Therefore, the distance, i.e., the static electrical capacity between the two electrodes, is changed and, therefore, a value of acceleration according to the change can be detected. Alternatively, a direct current electrical resistance between the two electrodes can be measured to determine a value of acceleration.
This type of known acceleration sensor, however, can only be used when the direction of acceleration is constant and when the capacity change is relatively large, since it is relatively difficult to detect the change in the static electrical capacity between the two electrodes.
Another known acceleration sensor also includes a cantilever beam bendable in accordance with an acceleration and a strain gauge mounted on the beam, so that a strain of the cantilever, i.e., a change in the electrical resistance of the strain gauge, corresponding to a value of acceleration exerted on a weight attached to a free end of the cantilever is detected. The cantilever may be made of a silicon substrate.
Assuming that an acceleration is now exerted in a direction in which the beam is bent and the weight having a mass (m) is subjected to a force, F=m.alpha.. In this case, a value of the electrical resistance will be represented as follows, on the basis of a stress T. EQU .vertline..DELTA..rho..vertline.=.vertline..rho..vertline..pi..vertline.T.v ertline.
Where, .DELTA..rho.: change of specific resistance due to stress PA1 .rho.: specific resistance without strain PA1 .pi.: piezo resistance coefficient
A semiconductor acceleration sensor is used to detect a value of acceleration on the basis of the change in this resistance.
In this type of known acceleration sensor, however, the cantilever beam possibly may be bent in other directions or torsioned according to the direction of an acceleration. In this case, an accurate detection of a value of acceleration in a particular direction, cannot be obtained.